docs: def _vec(A):
      """
      Linear operator _vec() from Wiktorsson2001 p478
      Args:
        A: a rank 3 array of shape N x m x n, giving a matrix A[j] for each
        interval of time j in 0..N-1
      Returns:
        array of shape N x mn x 1, made by stacking the columns of matrix A[j] on
        top of each other, for each j in 0..N-1
      """
      N, m, n = A.shape
      return A.reshape((N, m*n, 1), order='F')